1. Field of the Invention
The present invention relates to a novel transition metal compound, and more specifically, it relates to a novel transition metal compound having two transition metal atoms in one molecule and a method for polymerizing an olefin by the use of the same.
2. Description of the Related Art
It is known that a transition metal compound having a cyclopentadienyl group, an indenyl group, a fluorenyl group or their derivative as a ligand, i.e., a so-called metallocene compound can be used together with a co-catalyst such as an aluminoxane to polymerize an olefin, whereby a polyolefin can be prepared.
Japanese Patent Application Laid-open Nos. 19309/'83 and 35008/'85 disclose a method for polymerizing or copolymerizing an olefin in the presence of a catalyst comprising a metallocene compound and an aluminoxane.
In Japanese Patent Application Laid-open Nos. 130314/'86 and 66214/'89, it is described that a catalyst comprising a metallocene compound having a bridged ligand and an aluminoxane can be employed to prepare a poly-.alpha.-olefin having a high isotactic degree.
In Japanese Patent Application Laid-open Nos. 41303/'90, 274703/'90 and 274704/'90, it is described that a catalyst comprising a metallocene compound having mutually asymmetric and bridged ligands and an aluminoxane can be used to prepare a syndiotactic poly-.alpha.-olefin.
On the other hand, some catalyst systems containing no aluminoxanes as the co-catalysts have been reported, since it was suggested that an active species of the above-mentioned catalyst, i.e., the so-called Kaminsky type catalyst is a transition metal cation which can be represented by [Cp.sub.2 MR].sup.+ (wherein Cp is a cyclopentadienyl derivative; M is Ti, Zr or Hf; and R is alkyl).
As described in J. Organometall. Chem. 347, C9 (1988), Taube et al. have succeeded in the polymerization of ethylene by the use of a compound represented by [Cp.sub.2 TiMe(THF)].sup.+ [BPh.sub.4 ].sup.- (wherein Me is a methyl group; Ph is a phenyl group, and THF is tetrahydrofuran).
Jordan et al. have suggested in J. Am. Chem. Soc., 109, 4111 (1987) that a zirconium complex represented by [Cp.sub.2 ZrR(L)].sup.+ (wherein R is a methyl group or a benzyl group; and L is a Lewis base) functions to polymerize ethylene.
In Japanese Patent Application Disclosure (Kohyo) Nos. 501950/'89 and 502036/'89, it is described that a catalyst comprising a cyclopentadienyl metal compound and an ionic compound capable of stabilizing cyclopentadienyl metal cation is used to polymerize an olefin.
Zambelli et al. have reported in Macromolecules, 22, 2186 (1989) that a catalyst comprising a combination of a zirconium compound containing a derivative of cyclopentadiene as a ligand, trimethyl aluminum and fluorodimethyl aluminum permits preparing an isotactic polypropylene.
Japanese Patent Application Laid-open No. 179005/'91 discloses an olefin polymerization catalyst comprising (a) a neutral metallocene compound, (b) an aluminum alkyl and (c) a Lewis acid.
In Japanese Patent Application Laid-open No. 41303/'90, there is described a method for preparing a polyolefin having a wide molecular weight distribution in the presence of a catalyst comprising two or more kinds of metallocene compounds and an aluminoxane, and in the catalyst for use in this method, a zirconium compound is mixed with a hafnium compound. In general, the hafnium compound has an extremely low polymerization activity, and therefore in order to effectively prepare the polyolefin having the wide molecular weight distribution, the polymerization is required to be carried out at a high temperature. In consequence, a problem such as the decline of steric regularity of the produced polymer has taken place sometimes.
Furthermore, in Japanese Patent Application Laid-open No. 35008/'85, it is described that a catalyst obtained by mixing two kinds of zirconium compounds having different structures is used to prepare polyethylene having a wide molecular weight distribution. However, such a known method using the zirconium compound is insufficient to effectively expand the molecular weight distribution.